The orthogonal frequency division multiplexing (OFDM) scheme is a well-known high-speed data transmission scheme for next-generation communication technologies. In a conventional OFDM receiver, after being demodulated to baseband and digitized by an analog-to-digital (A/D) converter, an OFDM signal is usually sampled at a low sampling rate, which cannot resolve the multipath to take advantage of time diversity.
In light of the short comings in conventional OFDM receivers, a novel OFDM-based receiver is described herein. It is based on the bandpass sampling technique combined with delta-sigma modulation to demodulate and digitize an RF signal to baseband in a form of high-sampling-rate bit streams. The down-converted in-phase and quadrature output signals are clocked at a high sampling rate, equal to the carrier frequency that modulates the OFDM signal. The in-phase and quadrature outputs are bi-level digital signals, having values of 1 or −1.
Because of the high-sampling rate of the demodulated in-phase and quadrature digital signals (generally having a period of sub-nanoseconds), it is possible to exploit the multipath diversity as the granularity of the sampling period can resolve different RF signal paths due to reflections at objects and buildings reaching the quadrature bandpass-sampling OFDM receiver. Blocks of N-point Discrete Fourier Transform (DFT) can be time shifted with respect to each other before conversion from time to the frequency domain by the DFT, and the resulting in-phase and quadrature complex Fourier coefficients from each DFT block can be maximal ratio combined (MRC) at each subcarrier to provide diversity gain.